1. Field of the Invention
The present invention relates to a measurement method and apparatus.
2. Description of the Related Art
A projection exposure apparatus has conventionally been used to manufacture a fine semiconductor device, such as a semiconductor memory and a logic circuit, using the photolithography technology. The projection exposure apparatus uses a projection optical system to project and transfer a pattern of a reticle onto a wafer.
The projection exposure apparatus is required to precisely transfer a reticle pattern onto a wafer at a preset magnification or reduction ratio, and use of a projection optical system having an extremely reduced aberration (or an excellent imaging characteristic). In particular, along with the recent rapid miniaturization of a semiconductor device, a transferred pattern is sensitive to an aberration of an optical system. There is a demand for a precise measurement of an optical characteristic of the projection optical system, such as a wavefront aberration, while the projection optical system is installed in the exposure apparatus. In order to improve the productivity and the economical efficiency, easy, rapid, and inexpensive measurements are important.
An interferometer, such as a Fizeau interferometer and a Twyman-Green interferometer, is known as one conventional measurement apparatus that measures a wavefront aberration of an optical system, and an exposure apparatus mounted with the interferometer is proposed. See Japanese Patent Laid-Open No. 2000-277412. However, the conventional interferometer, such as a Fizeau interferometer and a Twyman-Green interferometer, complicates a structure of the entire system, has a problem of a large size and an increased cost, and it is very difficult to actually install it in the exposure apparatus. Accordingly, it is proposed to install in an exposure apparatus a comparatively simply structured interferometer, such as a point diffraction interferometer (“PDI”) (Japanese Patent Laid-Open No. 2000-97666) and a linear diffraction interferometer (“LDI”) (Japanese Patent Laid-Open No. 2004-273748).
Nevertheless, the conventional interferometer or measurement apparatus cannot precisely measure a wavefront aberration of a projection optical system. For example, the LDI finds a primary wavefronts of the projection optical system in two measurement directions which has a correct relative relationship in a direction perpendicular to the slit's longitudinal direction, and measures a final wavefront (wavefront aberration of the projection optical system) using phase information of the mutual measurement directions. The reference wavefront used to calculate the primary wavefront utilizes a slit diffraction wavefront that is a wavefront generated by a fine aperture slit equal to or smaller than the diffraction limit. When the slit diffraction wavefront is not an ideal reference wavefront (or when a reference wavefront contains an error), a difference from the ideal reference wavefront causes a measured optical system to contain a wavefront error and an error factor in the wavefront measurement by the LDI.